This invention relates to a magnetic pick-up device mounted in an ignition distributor of an automotive vehicle, more particularly to a magnetic pick-up device including a magnetoresistor which produces a voltage pulse caused by the variation of resistance of the magnetoresistor. The magnetic pick-up device senses the positions of the teeth of an armature rotated with the distributor shaft, which represents synchronously the positions of the pistons in the cylinders and produces a voltage pulse for interrupting current flow in the primary winding of the ignition coil so as to fire the spark plugs at the correct time.
Conventionally, the mechanical ignition system and the electronic ignition system are known types used on an internal combustion engine for providing the spark that ignites the combustible air-fuel mixture in the combustion chamber. Each of the mechanical and electronic ignition systems is provided with an ignition distributor which makes and breaks the primary ignition circuit, as well as distributes high tension current to the proper spark plug at the correct time. Referring to FIGS. 1(a), 1(b), a typical breaker type ignition distributor 10 includes a housing 11, in which the distributor shaft 12 is rotatably mounted. A breaker cam 30 is connected to the distributor shaft 12 and rotated thereby. When the breaker cam 30 rotates, each lobe 31 of the breaker cam 30 passes under the breaker lever rubbing block 40, causing the breaker points 20 to separate, as best illustrated in FIG. 1(a). Since the breaker points 20 are connected in series with the primary winding of the ignition coil, current will pass through the circuit when said breaker points 20 are closed. When the breaker points 20 are open, the magnetic field collapses and a high tension voltage is induced in the secondary winding of the ignition coil by the movement of the magnetic field through the coil windings. It is understood that the usual design is to provide one lobe on the breaker cam for each cylinder of the engine. As a result, every revolution of the breaker cam will produce one spark for each cylinder of the engine. After the high tension surge is produced in the ignition coil by the opening of the breaker points, the current passes from the ignition coil to the central terminal of the distributor cap. The current will then jump the minute gap to the distributor cap electrode which, in turn, is connected by high tension wiring to the spark plug designated by the firing order of the engine. However, such a breaker point type ignition distributor suffers from the disadvantage of wearing of the breaker points. Therefore, periodic checks of timing and dwell are necessary.
Referring to FIG. 2, a typical electronic ignition distributor 1000 comprises a distributor shaft 1001 rotatably mounted therein. An armature 2000, with teeth 2001 equally spaced therearound, is connected to the distributor shaft 1001. The positions of the teeth 2001 of the armature 2000 represent synchronously the positions of the pistons of the cylinders. A magnetic pick-up device 5000, which is mounted in the ignition distributor 1000, comprises a permanent magnetic bar 4000 and a pick-up coil 3000 provided around the permanent magnetic bar 4000. As the armature 2000 rotates, each tooth 2001 of the armature 2000 passes over the permanent magnetic bar 4000 of magnetic pick-up device 5000 and through the magnetic field of said permanent magnetic bar 4000, producing an induced electromotive force. Therefore, a voltage pulse will be created in the magnetic pick-up device 5000 each time a tooth 2001 of the armature 2000 passes said magnetic pick-up device 5000. The voltage pulse is then amplified and built up for firing a spark plug at the correct time. The magnetic pick-up device 5000 has replaced the function of the breaker points, and unlike the breaker points, shows no signs of wear. Therefore, periodic checks of timing and dwell are not necessary. Ignition maintenance is reduced to inspection of wiring, and cleaning and replacing of spark plugs as needed. However, the induced electromotive force caused by the induced current flowing in the pick-up coil 3000 will vary due to the low responsiveness of the pick-up coil 3000 when the engine speed is varied from high to low or vice versa. When the engine speed is high, the value of the voltage pulse is large, and when the engine speed is low, the value of the voltage pulse is small. Thus, the output value of voltage pulse is unstable. In addition, the the magnetic pickup device including the permanent magnetic bar 4000 and the pick-up coil 3000 is bulky in size.